The AH superfamily was identified in 1997 when Holm and Sander recognized the structural and mechanistic similarities within adenosine deaminase (ADA), phosphotriesterase (PTE), and urease (URE) [1]. The structurally characterized members of this superfamily fold as a distorted (p/a)8-barrel with an active site that is perched at the C-terminal end of the (J-barrel [2]. The active sites contain either a binuclear metal center or one of two possible mononuclear metal centers. The substrate recognition elements are formed by the conformations of the eight loops that connect the eight p/a-fragments. Most reactions catalyzed by the AH superfamily are hydrolytic but members have been that catalyze decarboxylation, isomerization, hydration, and retroaldol reactions. Members of the AH superfamily have been found in every sequenced genome; the current estimate is that the superfamily contains > 13,000 unique sequences. About 0.5% of the enzymes in the protein universe belong to the AH superfamily. Metal centers. The most common metal center within the AH superfamily is binuclear where the two metal ions (Mn2+, Fe2+, or Ni2+) are ligated to the protein through electrostatic interactions with six amino acid residues. The a-metal (Ma) is coordinated to two His residues at the end of B-strand 1 and an Asp at the end of B-strand 8; the B-metal (M(b)) is coordinated to two His residues at the ends of B-strands 5 and 6. The metal ions are bridged to one another by a carbamate functional group formed by post-translational addition of CO2 to the -amino group of a Lys at the end of B-strand 4. The metals are also bridged by a hydroxide from solvent. Variations on this theme include a Glu (at the end of either P-strand 3 or 4) as the bridging ligand and the substitution of an Asp for one of the His residues at the end of P-strand 1. In the most common mononuclear metal center a single divalent cation is bound to the Ma-site. The divalent cation is coordinated to the two conserved His residues at the end of P-strand 1, the His at the end of P-strand 5, and the invariant Asp at the end of B-strand 8; a water molecule is coordinated to the metal ion and to the His at the end of B-strand 6 and the Asp at the end of p-strand 8. A different mononuclear metal center is located at the Mp-site where the divalent cation is coordinated to His residues from B-strands 5 and 6 and another highly variable residue that can be either Cys or Glu; the coordination scheme is completed by a water molecule that is also hydrogen bonded to the Asp at the end of P-strand 8. Reactions catalyzed. More than 13,000 unique sequences have been identified in the AH superfamily. The sequences have been segregated into 9 major groups by the Superfamily/Genome Core, broadly represented by 24 clusters of orthologous groups (COGs) by the NCBI. More than 40 different reactions have been characterized for members of the AH superfamily; the number of reactions that remain to be discovered likely exceeds 100. The majority of the known reactions are hydrolytic reactions with carboxylate esters and amides and phosphate esters as substrates. The hydrolytic reactions also include deamination of aromatic amines. In addition, members of the AH superfamily catalyze decarboxylation.